1. Field of the Invention
This invention relates to the purification of gas streams and more specifically to the removal of vinyl chloride from a gaseous stream containing the same.
2. Description of the Prior Art
In industrial processes, it is desired to remove vinyl chloride gases from gas streams passed to the atmosphere. Due to the health hazards associated with this gas which have recently become known and due to regulatory activity on the subject of vinyl chloride (i.e., VCM) by a number of governmental regulatory agencies, there exists a need to remove vinyl chloride more completely from such gas streams. For example, the Occupational Safety and Health Administration has established an exposure limit of 1 ppm vinyl chloride monomer averaged over any 8 hours period; and a ceiling of 5 ppm vinyl chloride monomer averaged over any period not exceeding 15 minutes. Moreover, it is anticipated that the Environmental Protection Agency will also issue a regulation dealing with the emissions of vinyl chloride.
The presence of vinyl chloride in the atmosphere is, therefore, of major concern both to manufacturers of vinyl chloride monomer and to those manufacturers who convert the vinyl chloride monomer into polyvinyl chloride, a mainstay in the plastics industry. Vinyl chloride concentrations in vent gas streams as much as several hundred parts per million in large volumes of air flows may be expected to result from commercial processes, generally continuously operated, for production of vinyl chloride monomer. In addition, the normally batchwise process which is commercially employed to produce polyvinyl chloride will typically result in still larger flows of gases containing vinyl chloride. Thus, depending on the nature of operations at a given facility, the volumetric flow rate of vent streams containing vinyl chloride may range up to about 100,000 standard cubic feet of gases per minute. A process for removing vinyl chloride from gases over such a wide range of flow rates is therefore required.
While absorption of vinyl chloride by solids, such as charcoal, silica gel or molecular sieves may be employed to remove vinyl chloride gas from such vent streams, the high pressure drop associated with beds of solid particles is a serious disadvantage when handling large volumes of air and the loading and unloading of such beds is very awkward. Vapor phase reactions usually require use of high temperatures, a catalyst and/or photoinduction to cause vinyl chloride reactions to proceed, and such conditions are not favorable to removal of low vinyl chloride concentrations. Also, absorption of vinyl chloride in organic solvents such as mineral oil, alcohols or chlorinated solvents is disadvantageous due to the solvent's substantial vapor pressure which results in solvent vapors which must be recovered to prevent its passage to the atmosphere as an added pollutant. Thus, it would be desirable to employ an aqueous absorption system.
There have previously been developed processes wherein gas containing high concentrations of vinyl chloride are passed to an aqueous medium for reaction of the vinyl chloride to form chloroacetaldehyde. See, for example, U.S. Pat. No. 1,806,285 (issued in 1931 to Ernst et al.) and U.S. Pat. No. 2,060,303 (issued in 1936 to Groll et al.). See also British Pat. No. 299,722 and British Pat. No. 299,319. However, none of the foregoing processes effectively removes very low concentrations of vinyl chloride monomer from gases. Moreover, chloroacetaldehyde is also a severe health hazard and is a suspected carcinogen. Thus, conversion of vinyl chloride to chloroacetaldehyde only raises additional pollution problems.